Polar OwnIndex is not a reliable indicator of aerobic training status.

Polar watches with heart rate monitoring function have become popular among recreational and professional athletes. In addition to monitoring functions, they calculate a specific index called OwnIndex which is claimed to measure aerobic training status. The current research attempted to shed light on the factors determining the OwnIndex. In Study 1, OwnIndex calculated by the RS-400 Polar watch was estimated using anthropometric (gender, age, height, weight), cardiovascular (resting HR, RMSSD), and exercise-related (maximal oxygen uptake, self-reported physical activity) data of 45 young adults. In Study 2, the OwnIndex was measured in 21 young adults twice, first with self-reported physical activity set to the lowest, then to the highest value. In the regression analysis (Study 1), the only significant predictor of OwnIndex was self-reported physical activity (R² = 0.883; β = 0.915, p < 0.001). A significant difference with a large effect size (t(20) = -16.657, p <0.001, d = 3.635) and no significant correlation (r = -0.32; p = 0.155) were found between the OwnIndices calculated with different levels of activity in Study 2. As anthropometric and cardiac variables play a practically negligible role in the calculation of the OwnIndex, it cannot be considered an appropriate measure of aerobic fitness.